Conventional well drilling technologies, particularly for drilling oil wells, involve the use of a rotatable drill bit fastened to the end of a drill string. The bit is attached to the string by a threaded pin (or dome) on the bit that is received in a threaded `bit box` at the end of the string. The entire drill string, typically made up of a large number of threadably connected lengths of drill pipe, is rotated at the derrick to turn the bit in the hole. A wide variety of bits have been developed for use in different geological formations, such as the rock bits available from Smith International of Houston, Tex., USA.
A variety of drilling tools have been developed for attachment between the drill string and the bit to stabilize the bit in conventional straight-line drilling. U.S. Pat. No. 5,474,143 issued Dec. 12, 1995 to Smith International discloses a reaming drill bit stabilizer adapted for use in conventional drilling, in which the drill string is rotated to turn the bit. As disclosed therein, that reaming stabilizer is particularly suited to ensuring that the drill bit is stabilized and drills in a substantially straight line. In some circumstances, however, it is desirable to drill a well that deviates from a straight line.
Mud motors are particularly suited to directional drilling, in which the well bore is deliberately made to deviate from a straight line. In some formations, for example, it is desirable to drill horizontal wells, or a number of horizontal wells that are generally parallel to one another. Mud motors are typically capable of directional drilling because the motor is not straight. An example of such a motor is shown in U.S. Pat. No. 4,667,751 issued May 26, 1987 to Smith International. The bend in the mud motor may be oriented towards any wall of the well bore by turning the drill string. Once the motor is appropriately oriented, the motor may be operated to turn the bit, sliding the string forward as the well is excavated. The housing of the mud motor may be protected by stabilizers (which may be known as concentric stabilizers) which are not rotated by the motor, but may rotate with the whole motor for example when the motor is being oriented (see for example U.S. Pat. No. 4,667,751 issued to Smith International May 26, 1987). Concentric stabilizers are available, for example, for use with the DYNA-DRILL.TM. brand motors available from Smith International. Such stabilizers are typically slightly under-gage (smaller in diameter than the bit), since they serve to protect and in some cases to orient the motor.
Bits designed for drilling are generally not suited to reaming out a previously drilled bore. If they are to be used for reaming, such bits should generally be operated with reduced weight on the bit and at relatively low rpm., which may raise the cost and reduce the efficiency of drilling.